The invention relates to a television camera tube. The tube comprises, in an evacuated envelope, an electron gun. The electron gun generates an electron beam which, during operation of the tube, is focused to form a spot on a photosensitive target.
The electron beam is moved to scan the target. The electron gun, viewed in the direction of propagation of the electron beam, comprises successively a cathode, a grid, an anode and a cylindrical electrode having a diaphragm. Between the cathode and the anode, a crossover is formed in the electron beam. A part of the anode extends substantially perpendicularly to the electron beam, and has an aperture covered with a first metal foil -0-. on the side facing the target. The foil has an aperture at the area of the electron beam. The aperture in the foil has a diameter not more than 0.15 mm and not less than the diameter of the electron beam at that area.
Such a television camera tube is known from U.S. Pat. No. 3,928,784 which is incorporated herein by reference. A potential distribution is formed on the target by projecting an optical image on it. By scanning the target with the electron beam, the target provides signals corresponding to the optical image.
The photosensitive target usually consists of a photoconductive layer on a signal plate. The photoconductive layer may be considered to be composed of a large number of picture elements. Each picture element may in turn be considered as a capacitor to which a current source is connected in parallel whose current is substantially proportional to the light intensity on the picture element. The charge on each capacitor thus decreases linearly with time when the light intensity is constant.
As a result of the scanning, the electron beam passes through each picture element periodically and again charges the capacitor. This means that each picture element is periodically brought to the potential of the cathode. The quantity of charge which is necessary periodically to charge each capacitor is proportional to the light intensity on the picture element in question. The associated charging current flows to the signal plate via a signal resistor, all picture elements having the signal plate in common. As a result, a varying voltage is produced across the signal resistor, which voltage as a function of time represents the light intensity of the optical image as a function of the position of each picture element.
A television camera tube having the described operation is called a vidicon. As already described, each picture element is periodically brought to the cathode potential (zero volts). As soon as this potential is reached in a picture element the electrons of the electron beam can no longer reach the picture element. The electrons' velocities are reduced to zero, after which they are accelerated in the reverse direction.
A number of these reflected electrons form the so-called return beam which is deflected like the primary (scanning) electron beam. It has been found that at certain instants, the return beam can pass through the apertures in all the electrodes of the electron gun and can reach the space between the cathode and the anode. Many electrons have just insufficient energy to reach the cathode (which has a potential of zero volts), and they are then accelerated once again in the reverse direction. These electrons together constitute a secondary electron beam. Together with the primary electron beam, the secondary electron beam scans the photoconductive layer, but offset from the primary electron beam. The offset depends, inter alia, on the distance between the primary beam and the secondary beam in the aperture in the anode. As a result, the interfering signal is produced which is visible in the picture to be displayed.
In order to reduce the detrimental effect of the return beam, the anode in U.S. Pat. No. 3,928,784 is provided with a metal foil. At the area of the electron beam, the metal foil has an aperture with a diameter which is not more than 0.150 mm and not less than the diameter of the electron beam at that area. The diameter of the electron beam is the diameter of the smallest beam cross-section at that area. By choosing the aperture in the anode to be as small as possible, an important part of the return beam is intercepted by the anode without intercepting the primary electron beam. However, the anode does not intercept the primary electron beam. In practice it has been found that the measure described in U.S. Pat. No. 3,928,784 does reduce the interference resulting from the return beam, but does this to an insufficient extent.